Grace Baldonasa wrote:
> /*
> * convert a sampling rate into our full speed format (fs/1000 in Q16.16)
> * this will overflow at approx 524 kHz
> */
> inline static unsigned get_usb_full_speed_rate(unsigned int rate)
> {
> return ((rate << 13) + 62) / 125;
> }
>
> I have no clue as to why rate was shifted by 13 then added by 62 then
> divided by 125.
The parameter is in Hz. To convert to the number of samples per USB
frame, we have to divide by 1000. To convert to a 16.16-fixed-point
number, we have to multiply by 2^16. This would result in "rate *
0x10000 / 1000". Factoring out 8 yields "rate * 0x2000 / 125". The
multiplication by a power of two can be replaced by a bitwise left
shift. The "+62" prevents the division from rounding down.
HTH
Clemens
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